4. JAVA IntroClass

7/30/2019 4. JAVA IntroClass

1/28

Introduction to Class

Object-Oriented Programming


2/28

Module Introduction

1. Classes and Objects

2. Instance variables

3. Methods

4. Initializer

FPT- APTECH 2/28FPT- APTECH 2/28


3/28

#1 Classes and Objects

Class declaration

Constructors

Creating objects

FPT- APTECH 3/28


4/28

Creating Class

The template or blueprint from which

objects are actually made.

Syntax:

class ClassName{//definition of class

}

Al l cod e that you wr i te in Java is ins ide a

class.

Classes can be built by extending other

classes

FPT- APTECH 4/28


5/28

Constructor - how to instance an object?

Same name as the class.

A class can have more than one constructor.

A constructor can take zero, one, or more parameters.

Has no return value.

Always called with the new operator.

Use for initializing variables and invoking any methods thatmay be for initialization.

If a class does not have an explicit constructor, a defaultconstructor (no-argument) is provided.

FPT- APTECH 5/28


6/28

Name of class

1. Use Naming rules

2. Always captain first letter

3. Use Noun for naming

Data member

What data does it need to

know?Constructor

Define the way to

instance object

1. Same name as the

class name

2. Like function in C but

have no return value

Methods

What can it do?

1. Like function in C

2. Use Verb for naming

3. Always lower first letter

Dissecting the Rectangle class

FPT- APTECH 6/28


7/28

Creating objects (instance object)

An object must be created before using in a program.

1. Declare a variable to store the object reference.

1. Objects can only be manipulated via references

2. Creating an object.1. Using the newoperator in conjunction with a call to a constructor

FPT- APTECH 7/28


8/28

#2 Data Member

Instance Variables

Class Variables

FPT- APTECH 8/28


9/28

Instance variables

The values of those variables

define the state of object.

All instances of a class have

the same instance variables May have different values

inside them.

Has an access modifier

associated with it.

FPT- APTECH 9/28


10/28

Example of Instance variables

recA

length = 3.0

width = 4.0

recB

length = 6.4

width = 4.7

FPT- APTECH 10/28


11/28

Class Variables (Static Variables)

Variable that is accessed

without using an object of a

class.

Declare using the statickeyword.

Only one copy of a static

variable is shared by all the

objects of the class.

Change in the value of static

variable is reflected by all the

objects of the class.

FPT- APTECH 11/28


12/28

Example of Static Variables

FPT- APTECH 12/28


13/28

#3 - Methods

Definition

Instance Method

Calling method and Passing Arguments by Value.

Calling method and Passing Arguments by Reference.

Static Methods

Variable Argument Methods

FPT- APTECH 13/28


14/28

A method is defined as the actual implementation of an operation

on an object.

Syntax:

access_specifier modifier datatypemethod_name (parameter_list)

{//body of the method

}

The method name should begin with

a lowercase verb and follow with

a proper noun.

Method

FPT- APTECH 14/28


15/28

Instance Method

A function defined in a class

Invoked by an instance object

and can access instance

variables

Provide a mechanism for

accessing the private data

stored in an object

FPT- APTECH 15/28


16/28

Invoking Methods

Using the '.' operator

Syntax:

.

FPT- APTECH 16/28


17/28

Calling method and Passing Arguments by Value.

Value from calling method is passed as an argument to the

called method.

Any change made to that passed value in the called method

will not modify the value in the calling method.

Variables of primitive types (int, float ..) are passed by value.

FPT- APTECH 17/28


18/28

Example of Calling method and Passing Arguments by

Value.

FPT- APTECH 18/28


19/28

Calling method and Passing Arguments by Reference.

Called method to change the value of the parameter passed

to if from the calling method.

When references are passed as parameters, the caller can

change the values stored but not the reference variables.

FPT- APTECH 19/28


20/28

Example of Calling method and Passing Arguments by

Reference

FPT- APTECH 20/28


21/28

Static Methods

Methods that do not operate on objects.

Can be called without creating any objects of the class.

Call static method, supply the name of the class.

Declared using the static keyword.

Can directly refer only to static variables and other static

methods of the class.

Cannot refer to non-static methods and variables

FPT- APTECH 21/28


22/28

Example of Static Methods

FPT- APTECH 22/28


23/28

static methods usage

When a method doesn't need to access the object state

When a method only needs to access static fields of the

class.

FPT- APTECH 23/28


24/28

Variable Argument Methods

Allow calling a method with variable number of arguments.

FPT- APTECH 24/28


25/28

#4 - Initializer

Concept

Object Field Initializer

FPT- APTECH 25/28


26/28

Concept of Initializer

Initializers are small pieces of code embedded in curly

braces that perform initialization:

class block

class field

object block

object field

class Test {

static boolean b;// Automatic

static double d=1.2; // Explicitstatic {

System.out.println (Block");

System.out.println (d);

}

}

FPT- APTECH 26/28


27/28

Object Field Initializer

Object field initializers

also known as instance

initializers

Are declared inside a

class, but outside the

constructor and method

definitions.

public class A {//instance field initializerfloat price=9.50F;String str1;String str2;//instance block initializer

{str1 = AAA;str2 = BBB;

}public static voidmain(String[] args) { }

}

FPT- APTECH 27/28


28/28

Thats about all for today!

Thank you al l for yo ur attent ion and patient !

Classes and Objects

Instance variables

Methods

Initializer

Documents

4. JAVA IntroClass